Reliability of equipment is often reached by ensuring redundancy, i.e. by providing to the equipment a backup portion ready for being activated when the main portion of the equipment fails. Optical networks carry a vast amount of traffic and therefore they tend to use redundancy of optical equipment (usually, optical module/card redundancy).
Currently available redundancy solutions for optical networks suffer from the fact that due to their intrinsic fiber structure, both an active and a redundant card (module) is to be connected to the fiber line of interest by some optical equipment. The use of simple optical splitters dictates utilizing the so-called full (1:1) redundancy, which is double-priced by definition. However, when other redundancy schemes are utilized, it requires complex and thus very expensive optical switching matrices.
One commonly known today's solution using the 1+1 redundancy and regular optical Y-type splitters (combiners) is shown in FIG. 1. Each splitter is connected to a telecommunication optical line L that may be further connected to an optical data network (OND). For example, cards 1 and 1′ form the 1+1 redundancy configuration connected to a splitter S1 and serving line L1; any other pair of the cards in the example is provided with its own splitter and serves its specific line.
The main drawback of this solution is its price, since the full redundancy means just doubling of the number of cards. Therefore, there is an economic motivation to move to N+1 redundancy in order to reduce the optical network cost.
An example of another presently available redundancy solution is shown in FIG. 2a. In the illustrated N+1 solution, N main cards marked C1 . . . CN are protected by a single redundant card marked CN+1 with the aid of an optical matrix M. Each of the N main cards by default serves its assigned optical communication line (L1, L2, . . . LN) which may lead, for example, to respective N optical data networks ODN. When one of the N main cards becomes unavailable (see card C2 in FIG. 2b), the optical matrix M switches the backup card CN+1 to the optical network that was connected to the unavailable card: in our example, card#2 (C2) is malfunctioned, and its matching optical communication line L2 is switched to the redundant card CN+1).This N+1 scheme suffers both from high price of the optical matrix M and from its attenuation, which is much greater than that of a regular splitter.
ITU-T standard Recommendation G.983.1, in its Annex IV, recommends a number of redundancy implementations for optical access networks, and mentions that the recommended implementations are considered economic. However, all the configurations recommended in the standard comprise optical splitters and various combinations thereof which actually form optical matrices.
Another example of a complex optical switch matrix is described in US2002136484A, which relates to a multi-stage non-blocking optical switch matrix having failure protection.
It should be noted that the cost of optical switching matrices is extremely high and cannot be compared, even by order, with costs of complex switching equipment in non-optical systems (Say, an optical matrix of the switching ability 1:16 presently costs 60 times more than an electrical matrix of the same switching ability).
A non-optical switching system is described in US2002118581A that defines a memory having flexible column redundancy and flexible row redundancy comprises a multi-column stick configuration; each column stick comprising a plurality of data lines. Further, the memory has a multi-row stick configuration, with each row stick comprising a plurality of data rows. Positioned on either side of the memory are redundant column sticks each comprising a plurality of data lines. Positioned above and below the memory are redundant row sticks, each comprising a plurality of data rows. A column redundancy control identifies a faulty operating column stick in the memory and generates a column shift control signal to a column shift multiplexer that responds to the column shift control signal to substitute in the memory a redundant column stick for the identified faulty operating column stick. Further, the memory comprises a row redundancy controller that identifies a faulty operating row stick in the memory to generate a row shift control signal to a row shift multiplexer. The row shift multiplexer responds to the row shift control signal to substitute in the memory a redundant row stick for the identified faulty operating row stick in the memory. However, such or similar operations are impossible to perform in optical networks at the price of non-optical switching systems.